1. Field of the Invention
The present invention relates to a light beam condensing apparatus to introduce a condensing spot of a laser beam onto an optical recording medium such as optical disks, and to a method of driving the optical recording medium by applying the apparatus.
2. Description of the Prior Art
There has been frequently studied a technique to improve recording density of an optical disk so as to provide a large capacity optical disk. Thus, it has been known that reduction of a condensing spot diameter of a laser light beam for recording and regeneration is very effective in the improvement of the recording density, and mean surface recording density substantially increases while being inversely proportional to the square of the condensing spot diameter dSPOT. The condensing spot diameter dSPOT is proportional to a wavelength xcex of a laser to be used, and is inversely proportional to numerical aperture NA of an objective lens serving as a condenser, as shown in the following expression (1):
dSPOT=kxc2x7(xcex/NA) xe2x80x83xe2x80x83(1)
where the proportional constant k is defined by a wave front distribution of light wave incident on the lens. According to the expression (1), there are available three ways to reduce the condensing spot diameter dSPOT, i.e., the first way of reducing the wavelength of the laser to be used, the second way of increasing the numerical aperture of the objective lens serving as the condenser, and the third way of using super resolution in a condensing optical system.
A description will now be given of a method for providing a small condensing spot diameter by utilizing the super resolution in the condensing optical system. This method has been often disclosed in articles such as 1) Yamanaka et al., xe2x80x9cHigh Density Recording in Optical Disk by Super Resolutionxe2x80x9d in Optics, Vol.18, No.12, (1989), or 2) H. Ando, xe2x80x9cPhase-Shifting Apodizer of Three or More Portionsxe2x80x9d in Japanese Journal of Applied Physics, Vol.31, (1992). In these methods disclosed in the articles, it is possible to reduce the condensing spot diameter on the basis of the same principle, as shown in FIGS. 1 and 2.
FIG. 1 shows a configuration of an optical system of a conventional super resolution optical head as an example. In FIG. 1, reference numeral 101 means a laser oscillator, 102 means a collimate lens, 103 is a beam forming prism, 105 is an objective lens, 106 is a recording medium, and 107 is a shading plate.
A description will now be given of the operation. Laser light from the laser oscillator 101 serving as a light source is collimated through the collimate lens 102 and the beam forming prism 103, resulting in parallel light. A laser beam 104 serving as the parallel light is focused and condensed by the objective lens 105 on a recording surface of the recording medium 106. Here, the shading plate 107 is disposed across the laser beam 104 so as to partially shade the laser beam 104. At the time, the condensing spot diameter dSPOT of the laser beam 104 is varied according to a position and a shape of the shading plate 107, that is, a width and a length thereof.
A description will now be given of the principle in the reduction of the condensing spot diameter by the super resolution with reference to FIG. 2. As shown in FIG. 2, in case the shading plate 107 is longer than a beam diameter D of the collimate beam 104, a condensing spot diameter dSPOTt in a cross direction of the shading plate is defined as a ratio of the beam diameter D to the width xcex94W of the shading plate 107 if the width of the shading plate 107 is defined as xcex94W. Further, a condensing spot diameter dSPOTr in a longitudinal direction of the shading plate 107 is substantially irrelevant to the width xcex94W. Here, as the width xcex94W becomes larger, sidelobes 108 in a condensing spot becomes higher while the condensing spot diameter dSPOTt of a mainlobe 109 becomes smaller.
FIG. 3 shows a relation between xcex94W/D and the condensing spot dSPOTt. As understood from FIG. 3, as xcex94W/D is more increased, the condensing spot diameter dSPOTt is more reduced, and concurrently intensity of the sidelobe is more increased. Since increase of the sidelobe causes an increase of crosstalk, it is impossible to allow the sidelobe to become so large. Here, xcex94W/D=0 if the shading plate 107 is not employed. At the time, if the condensing spot diameter is set to dSPOT0, it is possible to reduce the condensing spot diameter dSPOTt to 10% degree as compared with dSPOT0 when the sidelobe intensity can be in a range of 0.1 times the mainlobe or less. In such a way, it is possible to reduce the condensing spot diameter with the constant laser wavelength xcex and the constant numerical aperture NA of the lens by shading a vicinity of an intermediate portion of the collimate beam in a super-resolution optical head. When the shading plate 107 is coplanarly rotated by 90xc2x0, the condensing spot diameter dSPOTt is left as it is dSPOT0, and the condensing spot diameter dSPOTr is reduced.
As set forth above, the principle of the super resolution utilizes the character of focusing light wave that it is possible to vary the intensity distribution at the condensing spot by modulating a wave front of the collimate beam 104 on an entrance surface of the objective lens 105. That is, the shading plate 107 shown in FIG. 2 corresponds to space modulation which is performed so as to set an amplitude distribution of the collimate beam 104 on the entrance surface of the objective lens 105 to zero in the vicinity of the intermediate portion of the collimate beam 104. Accordingly, laser power at a shaded portion is lost.
Further, on the basis of the principle of the super resolution, it is also possible to vary the intensity distribution of the condensing spot by modulating a phase distribution of the collimate beam 104 on the entrance surface of the objective lens 105. That is, it is possible to form a condensing spot shape by providing appropriate phase shift according to a position on the entrance surface of the objective lens 105. This method is employed in the article 2) as described before. In this case, the collimate beam 104 is not shaded so that there is no partial loss of the laser power due to the shading.
Alternatively, in another known technique, a distribution is caused in indexes of refraction in order to provide phase modulation to transmitted light. Assumed that there is difference xcex94n between the indexes of refraction sensed by the transmitted light at two portions of a modulation plate when light having the wavelength xcex passes through the modulation plate having a thickness of L. Consequently, in the light beam passing through both the portions, there is generated a phase difference xcex94xcfx86 expressed by the following expression (2):
xcex94xcfx86=2xcfx80(L/xcex)xc2x7xcex94n xe2x80x83xe2x80x83(2)
A phase of the transmitted light is modulated by the phase difference. It must be noted that a method of the phase modulation of the transmitted light should not be limited to a method to provide a difference in an optical path length by the difference in the indexes of refraction. It is similarly possible to provide the difference in the optical path length by varying the thickness of the modulation plate so as to perform the phase modulation of the transmitted light.
The recording density of the optical disk can be expressed by the product of recording density in a direction parallel to a recording track (i.e., track recording density BPI) and recording density in a direction perpendicular to the recording track (i.e., track density TPI). Therefore, it is possible to improve surface recording density of the optical disk by improving the BPI and the TPI, respectively. The conventional embodiment shown in FIG. 2 is provided to improve the BPI. For example, if a concentrically circular shading plate to shade the intermediate portion exclusively is employed instead of the shading plate 107 shown in FIG. 2, the condensing spot has a concentrically circular shape so that the mainlobe 109 is surrounded by the sidelobe 108. In this case, the condensing spot diameter of the mainlobe 109 can be reduced. Thus, it is possible to concurrently improve the BPI and the TPI by using the condensing spot.
As set forth above, the reduction of the condensing spot diameter by the super resolution is an effective technique to improve the recording density. According to the prior art, it is possible to provide a constant condensing spot diameter by varying the amplitude or the phase of the transmitted light by a fixed optical component such as the shading plate, or the phase plate. However, in the prior art, it is impossible to vary a parameter of the super resolution, that is, the modulation amount applied to the wave front of the collimate beam on the entrance surface of the objective lens during the operation in one condensing apparatus so as to dynamically vary the condensing spot diameter or the condensing spot shape of an optical disk unit.
On the other hand, in the current market, there are employed the optical disks compatible to the optical disk standard which is standardized by the ISO standard or the like. Most of these disks have a track pitch of 1.6 (xcexcm) and the track recording density of 25 (kbit/inch). Further, the large capacity optical disk has been developed in recent years, and the track pitch is more reduce and the track recording density is more increased if it is possible to provide the practical large capacity optical disk having more improved recording density than that of the conventional optical disk. Accordingly, a condensing spot diameter smaller than that in the prior art is required for recording and regenerating information. The condensing spot diameter can be provided by applying a shorter wavelength laser, an objective lens having larger numerical aperture, or the super resolution as described before. In this case, it is to be understood that the condensing spot diameter is designed so as to be adaptable to the track pitch or the track recording density of a newly developed large capacity optical disk.
Here, compatibility of an optical disk drive becomes a major issue. That is, in case the optical disk drive is provided with a function to drive both the newly developed large capacity optical disk and the optical disks based upon the conventional standard, there are the following three problems. The first problem relates to a tracking servo. A servo sensor signal for tracking is detected depending upon diffraction phenomena of the spot on the disk surface because of a guide groove, i.e., a periodic structure of a groove and a land on the optical disk. Hence, if the condensing spot is designed so as to be adaptable to a narrow-width track pitch, there is a drawback in that it is not possible to sufficiently provide a servo error signal for tracking when the conventional optical disk having a wide-width track pitch is driven.
The second problem occurs at a time to read an emboss signal. While information of the emboss signal is recorded on the optical disk in a form of a phase bit, the signal regeneration is performed depending upon a principle that condensed light is diffracted by the phase bit, and an amount of reflected light to be received by the detector is varied according to the presence or absence of the bit. Therefore, it is impossible to provide a sufficient variation rate by the diffraction in case the condensing spot diameter is too small with respect to the phase bit. As a result, there is another drawback in that reading accuracy is reduced or incapability of reading occurs due to a reduced regenerative amplitude of the emboss signal.
The third problem occurs when the information on the medium is erased in a rewritable optical disk. In optical disks which is recorded and erased by thermal energy of the condensed light such as magneto-optical medium, or phase varying medium, when a signal recorded on a low density medium having the wide-width track is erased by the condensing spot having a small diameter, it is impossible to erase an entire width of the recorded mark since an erasable width is narrow, resulting in an unerased portion. As a result, there is still another drawback in that the unerased portion is left as the crosstalk, and increases occurrence of regeneration error when the erasing and recording is repeated.
In view of the foregoing, it is an object of the present invention to provide a light beam condensing apparatus which can ensure compatibility of many kinds of optical recording media as a main function required in a medium interchangeable information recording apparatus, and a method of driving an optical recording medium by applying the light beam condensing apparatus.
It is another object of the present invention to attempt improvement of main performances required in the optical disk drive, such as improvement of data reliability and seek reliability, and extension of a disk receiving range by effectively using functions which can realized according to the present invention.
According to the first aspect of the present invention, for achieving the above-mentioned objects, there is provided a light beam condensing apparatus including an objective lens to condense a collimate beam so as to introduce a condensing spot of the collimate beam onto an optical recording medium, and modulation means for varying a shape of the condensing spot by modulating the collimate beam.
As stated above, in the light beam condensing apparatus according to the first aspect of the present invention, a laser beam emitted from a laser oscillator is transformed into the collimate beam by a collimate lens. The objective lens condenses the collimate beam to introduce the condensing spot of the collimate beam onto the optical recording medium. Further, the modulation means varies the shape of the condensing spot by modulating the collimate beam. Therefore, it is possible to use a small-diameter condensing spot with respect to a high recording density medium, and use a large-diameter condensing spot with respect to a low recording density medium.
According to the second aspect of the present invention, there is provided a light beam condensing apparatus including modulation means for varying a shape of a condensing spot by modulating a collimate beam, and control means for controlling the modulation means to vary the shape of the condensing spot.
As stated above, in the light beam condensing apparatus according to the second aspect of the present invention, since the control means is provided to control the modulation means, it is possible to control the modulation means.
According to the third aspect of the present invention, there is provided a light beam condensing apparatus to vary transmission factor of a collimate beam by modulation means so as to vary a shape of a condensing spot.
As stated above, in the light beam condensing apparatus according to the third aspect of the present invention, it is possible to vary the transmission factor of the collimate beam by the modulation means so as to vary the shape of the condensing spot. As a result, it is possible to adjust a diameter of the condensing spot according to high or low recording density of an optical recording medium.
According to the fourth aspect of the present invention, there is provided a light beam condensing apparatus to vary a phase of a collimate beam by modulation means so as to vary a shape of a condensing spot.
As stated above, in the light beam condensing apparatus according to the fourth aspect of the present invention, it is possible to vary the phase of the collimate beam by the modulation means so as to vary the shape of the condensing spot. Therefore, it is possible to reduce or extend a diameter of the condensing spot without shading the collimate beam.
According to the fifth aspect of the present invention, there is provided a light beam condensing apparatus to modulate a collimate beam so as to vary a shape of a condensing spot when a modulation plate is positioned in a direction perpendicular to an optical axis of the collimate beam, and to cease the modulation of the collimate beam when modulation means is positioned in a direction parallel to the optical axis of the collimate beam.
As stated above, in the light beam condensing apparatus according to the fifth aspect of the present invention, the modulation means is provided in a flat shape, and it is possible to modulate the collimate beam so as to vary the shape of the condensing spot when the modulation plate is positioned in the direction perpendicular to the optical axis of the collimate beam, and to cease the modulation of the collimate beam when modulation means is positioned in the direction parallel to the optical axis of the collimate beam. Therefore, the shape of the condensing spot can be switched over by simply rotating the flat modulation means.
According to the sixth aspect of the present invention, there is provided a light beam condensing apparatus in which modulation means is rotatably supported about an optical axis of a collimate beam when the modulation means is positioned in a direction perpendicular to the optical axis of the collimate beam.
As stated above, in the light beam condensing apparatus according to the sixth aspect of the present invention, the flat modulation means can be rotated about the optical axis of the collimate beam when the modulation means is positioned in the direction perpendicular to the optical axis of the collimate beam. Therefore, a shape of a condensing spot can be switched over as in the fifth aspect, and the condensing spot can be reduced in all directions by rotating the modulation means about the optical axis.
According to the seventh aspect of the present invention, there is provided a light beam condensing apparatus in which, when any one of modulation plates is positioned in a direction perpendicular to an optical axis of a collimate beam to vary a shape of a condensing spot, the other modulation plate is positioned in a direction parallel to the optical axis of the collimate beam.
As stated above, the light beam condensing apparatus according to the seventh aspect of the present invention includes the pair of modulation plates which are mounted in a substantially cross form, and when any one of the modulation plates is positioned in the direction perpendicular to the optical axis of the collimate beam to vary the shape of the condensing spot, the other modulation plate is positioned in the direction parallel to the optical axis of the collimate beam. Therefore, two kinds of condensing spot diameters can be switched over from one to another by positioning the one modulation plate and the other modulation plate in the direction perpendicular to the optical axis of the collimate beam.
According to the eighth aspect of the present invention, there is provided a light beam condensing apparatus including modulation means. The modulation means has a cylindrical body whose peripheral surface is provided with a modulation pattern, and a shape of a condensing spot is varied by positioning the modulation pattern of the peripheral surface across a course of a collimate beam.
As stated above, the light beam condensing apparatus according to the eighth aspect of the present invention includes the cylindrical modulation means whose peripheral surface is provided with the modulation pattern, and it is possible to vary the shape of the condensing spot by positioning the modulation pattern of the peripheral surface across the course of the collimate beam. Therefore, since a plurality of modulation patterns can be formed on the peripheral surface of the cylindrical body, it is possible to provide different types of the shapes of the condensing spot.
According to the ninth aspect of the present invention, there is provided a light beam condensing apparatus in which modulation means includes a pair of modulation plates, and when the respective modulation plates are positioned in a direction perpendicular to an optical axis of a collimate beam, the respective modulation plates are coplanarly positioned, and a modulation pattern is provided by the respective modulation plates.
As stated above, in the light beam condensing apparatus according to the ninth aspect of the present invention, the modulation means includes the pair of modulation plates, and when the respective modulation plates are positioned in the direction perpendicular to the optical axis of the collimate beam, the respective modulation plates are coplanarly positioned, and the modulation pattern is provided by the respective modulation plates. Therefore, it is possible to optionally select a modulation mode to provide no modulation by positioning the respective modulation plates in a direction parallel to the optical axis of the collimate beam, a first modulation mode to provide the collimate beam with first modulation by positioning so as to pass the collimate beam through a half surface of one of the respective modulation plates, and a second modulation mode to provide the collimate beam with second modulation by positioning so as to pass the collimate beam through a half surface of the other of the respective modulation plates.
According to the tenth aspect of the present invention, there is provided a light beam condensing apparatus including modulation means. The modulation means has a modulation plate which is provided with a plurality of modulation patterns, and is supported slidably in a direction perpendicular to an optical axis of a collimate beam, and the modulation means can position a desired modulation pattern in the plurality of modulation patterns across an optical path of the collimate beam.
As stated above, the light beam condensing apparatus according to the tenth aspect of the present invention includes the modulation means. The modulation means has the modulation plate which is provided with the plurality of modulation patterns, and is supported slidably in the direction perpendicular to the optical axis of the collimate beam, and the modulation means can position the desired modulation pattern in the plurality of modulation patterns across the optical path of the collimate beam. Therefore, it is possible to easily increase the number of the modulation patterns.
According to the eleventh aspect of the present invention, there is provided a light beam condensing apparatus including modulation means. The modulation means has a modulation plate which is formed by a plurality of optical components, and is disposed across an optical path of a collimate beam, and the modulation means varies a modulation pattern of the modulation plate by applying voltage to the optical components.
As stated above, the light beam condensing apparatus according to the eleventh aspect of the present invention includes the modulation means. The modulation means has the modulation plate which is formed by the plurality of optical components, and is disposed across the optical path of the collimate beam, and the modulation means can vary the modulation pattern of the modulation plate by applying the voltage to the optical components. Therefore, since the modulation pattern can be electrically varied, it is possible to accelerate a switching speed.
According to the twelfth aspect of the present invention, there is provided a light beam condensing apparatus in which a modulation pattern of modulation means is formed by a rectangular modulation section positioned at an intermediate portion of a collimate beam, and having a longitudinal side longer than a collimate beam diameter, and by a modulation section positioned at the other portion of the collimate beam.
As stated above, in the light beam condensing apparatus according to the twelfth aspect of the present invention, the modulation pattern of the modulation means is formed by the rectangular modulation section which is positioned at the intermediate portion of the collimate beam, and has the longitudinal side longer than the collimate beam diameter, and by the modulation section positioned at the other portion of the collimate beam. Therefore, it is possible to reduce or extend a diameter of a condensing spot in one direction by the modulation section at the intermediate portion.
According to the thirteenth aspect of the present invention, there is provided a light beam condensing apparatus in which a modulation pattern of modulation means is formed by a circular modulation section positioned coaxially with a collimate beam, and having a diameter smaller than a collimate beam diameter, and by a modulation section positioned at the other portion of the collimate beam.
As stated above, in the light beam condensing apparatus according to the thirteenth aspect of the present invention, the modulation pattern of the modulation means is formed by the circular modulation section positioned coaxially with the collimate beam, and having the diameter smaller than the collimate beam diameter, and by the modulation section positioned at the other portion of the collimate beam. Therefore, it is possible to reduce or extend a diameter of a condensing spot by the circular modulation section in all directions.
According to the fourteenth aspect of the present invention, there is provided a method of driving an optical recording medium by applying a light beam condensing apparatus. The method comprises the steps of setting a condensing spot shape of a condensing beam focused into a recording bit area of an optical recording medium to be adaptable to a track pitch, and performing recording and regeneration of the optical recording medium at the condensing spot having the set shape.
As stated above, in the method of driving the optical recording medium by applying the light beam condensing apparatus according to the fourteenth aspect of the present invention, track pitch information in the recording bit area is read by the condensing beam emitted from the light beam condensing apparatus to control modulation means of the light beam condensing apparatus depending upon the read track pitch information so as to modulate a collimate beam, and the condensing spot shape of the condensing beam focused into the recording bit area of the optical recording medium is set to be adaptable to the track pitch. Therefore, the condensing spot can be switched over according to the track pitch of the optical recording medium.
According to the fifteenth aspect of the present invention, there is provided a method of driving an optical recording medium by applying a light beam condensing apparatus. The method comprises the steps of detecting an amplitude of a tracking servo sensor signal at a time of a set condensing spot shape, adjusting the set condensing spot shape by controlling modulation means so as to maximize the detected amplitude, and performing recording and regeneration of the optical recording medium at the adjusted condensing spot.
As stated above, in the method of driving the optical recording medium by applying the light beam condensing apparatus according to the fifteenth aspect of the present invention, the amplitude of the tracking servo sensor signal at the time of set condensing spot shape is detected, and the set condensing spot shape is adjusted by controlling the modulation means so as to maximize the detected amplitude. Therefore, it is possible to absorb variation in the tracking servo signal which is different for each combination of the optical recording medium and a drive unit thereof.
According to the sixteenth aspect of the present invention, there is provided a method of driving an optical recording medium by applying a light beam condensing apparatus. The method comprises the steps of detecting an amplitude of a regenerative signal of an emboss signal before performing regeneration of the optical recording medium with a condensing spot, and controlling modulation means to maximize the detected amplitude so as to adjust a condensing spot shape to be optimal for information regeneration.
As stated above, in the method of driving the optical recording medium by applying the light beam condensing apparatus according to the sixteenth aspect of the present invention, it is possible to detect the amplitude of the regenerative signal of the emboss signal before performing the regeneration of the optical recording medium with the condensing spot, and control the modulation means to maximize the detected amplitude so as to adjust the condensing spot shape to be optimal for the information regeneration. Therefore, it is possible to absorb variation in an information regenerating signal characteristic which is different for each combination of the optical recording medium and a drive unit thereof.
According to the seventeenth aspect of the present invention, there is provided a method of driving an optical recording medium by applying a light beam condensing apparatus. The method comprises the steps of setting a condensing spot shape to have a large diameter by controlling modulation means of the light beam condensing apparatus before reading track pitch information in a recording bit area.
As stated above, in the method of driving the optical recording medium by applying the light beam condensing apparatus according to the seventeenth aspect of the present invention, it is possible to set the condensing spot shape to have the large diameter by controlling the modulation means of the light beam condensing apparatus before reading the track pitch information in the recording bit area. Therefore, it is possible to improve reading accuracy of the track pitch information in the recording bit area with respect to various types of optical recording media.
According to the eighteenth aspect of the present invention, there is provided a method of driving an optical recording medium by applying a light beam condensing apparatus. The method comprises the steps of setting a condensing spot shape in a large size by controlling modulation means of the light beam condensing apparatus before focusing on a control track area for reading track pitch information.
As stated above, in the method of driving the optical recording medium by applying the light beam condensing apparatus according to the eighteenth aspect of the present invention, it is possible to set the condensing spot shape in the large size by controlling the modulation means of the light beam condensing apparatus before the focus pulling in for reading the track pitch information in the recording bit area. Therefore, it is possible to improve stability of the focus pulling in.
According to the nineteenth aspect of the present invention, there is provided a method of driving an optical recording medium by applying a light beam condensing apparatus. The method comprises the steps of controlling modulation means of the light beam condensing apparatus according to a driving condition of the recording medium to switch a shape of a condensing spot which is focused on the optical recording medium.
As stated above, in the method of driving the optical recording medium by applying the light beam condensing apparatus according to the nineteenth aspect of the present invention, the light beam condensing apparatus is provided to emit a laser beam from a laser oscillator, transform the emitted laser beam into a collimate beam and condense the collimate beam so as to introduce the beam into a bit area of the optical recording medium. In the light beam condensing apparatus, it is possible to control the modulation means of the light beam condensing apparatus according to the driving condition of the recording medium so as to switch the shape of the condensing spot which is focused on the optical recording medium. Therefore, control can be made such that spot shape modification is provided at a time of seek and regeneration, and the spot shape modification is not provided at a time of recording.
According to the twentieth aspect of the present invention, there is provided a method of driving an optical recording medium by applying a light beam condensing apparatus. The method comprises the steps of, in case it is detected that sector read is not normal at a time of regeneration of the optical recording medium, controlling modulation means depending upon the detected signal so as to adjust a condensing spot shape, retry the sector read, and repeat the above steps until the sector read can be normally performed.
As stated above, in the method of driving the optical recording medium by applying the light beam condensing apparatus according to the twentieth aspect of the present invention, it is possible to, in case it is detected that the sector read is not normal at the time of regeneration of the optical recording medium, control the modulation means depending upon the detected signal so as to adjust the condensing spot shape, retry the sector read, and repeat the above steps until the sector read can be normally performed. Therefore, it is possible to improve such a possibility that a condition incapable of reading a signal can be avoided.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawing. It is to be expressly understood, however, that the drawings are for purpose of illustration only and are not intended as a definition of the limits of the invention.